1. Field of the Invention
This invention relates to a method of subjecting cement clinker burned in a rotary kiln to a reduction, by spraying the clinker with a reducing agent, and subsequently cooling it. The invention also relates to a plant for practicing the improved method.
2. Description of the Prior Art
A known method for producing white cement consists in effecting the reduction of the cement clinker inside the kiln. The reducing agent is added to the clinker at a high temperature, and water is sprayed on the clinker by means of nozzles immediately afterwards so as to effect a quenching. This method not only produces the well known effect on the whiteness of the cement due to quenching, but also prevents oxidation by ensuring that no air is admitted during the addition of the reducing agent to the clinker so that reoxidation is prevented.
This method has been found to produce a very favourable whiteness of cement in practice. Furthermore, the method has the advantage that the clinker is perfectly dry when leaving the kiln system, making extra drying unnecessary.
However, a drawback of the method is that a proportion of the water vapours generated by the water cooling enter the kiln together with the smoke from the combustion, even when means are provided for removing a maximum amount of water vapour by suction. This inevitable amount of water vapour results in an increased heat consumption in the burning step and thus a lower production of the rotary kiln than that which would be attained without the presence of the water vapour.
Commonly assigned U.S. Pat. No. 3,074,075 to Dano et al relates to such a treatment where material traveling through a rotary kiln is reduced in the kiln by directing a conical spray of a reducing agent upon an area of the inclined surface of the moving burned material with the axis of the spray normal to the inclined surface. The reduced material is cooled by directing a conical spray of cooling agent upon a closely adjacent area of the inclined surface of the material, with the axis of the spray normal to the inclined surface. The material burned is a raw material suitable for the production of white cement.
It is also known to effect water cooling of the clinker outside the rotary kiln. Usually, this step is performed by causing the clinker when discharged from the kiln to fall into a water bath from which it is rapidly removed, for example, by means of a drag chain. This method, although ensuring that the water vapour is prevented from entering the kiln, has two essential drawbacks. First, the whiteness obtained in practice is inferior to the whiteness obtained by the former method, and secondly, the clinker has a water content of 10-12% after the process. This requires a supplementary drying, i.e., a supply of extra heat. The water content also reduces the strength of the cement manufactured from such clinker because a certain amount of hydration is inevitable. I have invented a method and plant for producing cement in which the disadvantages of the prior art are avoided, and the advantages are successfully and usefully combined to produce an improved product.